In recent years, a semiconductor device incorporating a power semiconductor element has been improved in performance and reduced in size, and required to have higher heat dissipation performance than has been conventionally possible. A power semiconductor element in such a semiconductor device is required to have high heat dissipation characteristics to surrounding components or to the outside of the components, while ensuring insulation from the surrounding components. For this purpose, a ceramic substrate that combines thermal conductivity and insulation properties is often used in such a semiconductor device. However, a ceramic substrate has a disadvantage of being brittle and having poor processability.
Meanwhile, an insulating sheet has been increasingly used recently in a semiconductor device required to have heat dissipation characteristics. An insulating sheet is made of resin filled with an inorganic powder filler made of ceramic particles, for example. In a power module of WO 2012/073306 (PTD 1), for example, an insulating sheet arranged to cover the upper surface of a heat dissipation plate improves efficiency of thermal conduction from a lead frame to the heat dissipation plate while ensuring electrical insulation between the lead frame and the heat dissipation plate.
In addition, the power module of WO 2012/073306 has a configuration in which a power semiconductor element, the heat dissipation plate, the insulating sheet and the like are sealed with epoxy resin. In this power module, the thickness of the lead frame varies between a region in which the power semiconductor element is placed directly thereon and the other region for electrical connection to the outside. By using such a lead frame that varies in thickness between the regions, a long so-called creepage distance, which is the distance between the lead frame and the heat dissipation plate in a direction along the surface of the heat dissipation plate, for example, can be ensured, whereby the electrical insulation between the lead frame and the heat dissipation plate can be further improved.